1. Field of the Invention
This invention relates to a process for the production of high-purity mullites for use as heat-resistant structural materials.
2. Prior Art
Attention has been focused lately on the use of ceramics which have superb high temperature performance characteristics unparallelled with conventional metallic or organic heat-resistant materials.
Ceramics of oxide type are typically silicon nitride and silicon carbide and those of non-oxide type are typically alumina and zirconia. Even these ceramics materials are evaluated to be not entirely satisfactory to combat such extremely high temperature industrial applications as are and will be demanded by the ever growing high technology of today. For instance, when taking into account a temperature-dependent flexural strength, silicon nitride, silicon carbide and alumina have manageable critical temperatures of up to 1,200.degree. C., 1,300.degree. C. and 1,000.degree. C., respectively. However, there are in fact many fields of application where structural materials are exposed to much higher temperatures than just indicated. With this background in view, mullites come to the fore which are reputed for strong covalent linkage and improved temperature-dependent characteristics related to mechanical strength. Mullites are one of the well-known aluminosilicate minerals generally represented as 3Al.sub.2 O.sub.3 .multidot.2SiO.sub.2 --2Al.sub.2 O.sub.3 .multidot.SiO.sub.2, and usually in a spicular form found in porcelain and refractory manufactured from naturally occuring clay minerals. Typical processes for making the mullites include a sol-gel process in which alumina sol and silica gel are homogeneously mixed, the resulting mixture being adjusted in pH, dehydrated, gelled and finally calcined; a co-precipitation process in which a mixture of aqueous aluminum salt and silica sol is neutralized with aqueous ammonium, the resulting co-precipitate being filtered, washed, dehydrated, dried and calcined and; an alkoxide process in which alcoholate of aluminum and silicon are admixed, hydrolyzed with addition of water, hydrated, dried and calcined. Other known processes are spray-pyrolysis and hydrothermal synthesis. The non-crystalline product obtained is calcined at 1,250.degree.-1,400.degree. C. thereby producing a mullite having a mean crystalline particle size of 10 nm. The purity, particle size, particle size distribution and crystalline structure of a given mullite vary with the starting material and process employed and are influential upon moldability, sintering and product quality.
It has been believed that mullites being oxide-type ceramics undergo only appreciable deterioration by oxides in the air and have high temperature strength, hopefully leading to successful application for automotive engine and gas turbine component parts. This was however hindered by their relatively low tenacity and weak mechanical strength as a structural material.
In order to eliminate the above drawbacks of mullite products, it has been proposed amongst other studies to suppress the formation of a glass phase, or to further enhance the purity of mullite per se. As for an example, there is known a process in which kaolin with low silica/alumina contents is formed and calcined at a temperature above 1,600.degree. C., followed by evaluation of the glass phase in hydrogen fluoride.
The conventional processes are however somewhat complex and not fully immune to impurities contributory to glass phase formation.
It has therefore been considered important to find ways to preclude entry of alkali metal oxides and other detrimental impurities into the mullite system so as to eliminate glass phase formation and to allow calcination at a relatively low temperature thereby obtaining highest possible purity products.